Yes Biggin is busy, and the landing fees do mount up, but don't forget the journey times to and from the airport! This counts for a lot when you consider that you could be at the airfield anywhere from 4 to 6 times a week!!! Don't forget things like ground school, and just hanging around at the airport waiting for the weather to clear - if you have driven to Headcorn / Elstree you will be pretty frustrated if you cant fly!! And you may be able to fit in a flight after work if you learn locally whereas if you have an hour plus with the rush hour against you it is not going to happen!
An advantage of Biggin is that if you learn there, you are prepared to fly anywhere -it all prepares you for future flying. I learnt there, and was happy to go flying into airports like Schipol (V expensive but great fun), Rotterdam, Palma, and all sorts of others around UK and Europe. If you learn at a small field with only air/ground you will not get the same wide experience every time you train, especially the RT experience a busy approach/tower gives you.
The best thing I learnt (apart from the flying of course) was it is essential to learn at the nearest proper airport to you. End of!
In terms of trial lesson, well you can do a trial at as many places as you want, but you will find it well worth going and talk to each and every school / club at the airport - just walk in unannounced and see what you think of the treatment you get. Have a good look around, ask other people how it all works. Then have a chat with an instructor who would be the one to instruct you, they will be more than happy to show you the planes, expalin how things work and talk you through what you need. Do this on a rainy no fly day and you will have plenty of help!! Then once you have visited every school, weigh up how you feel and then book a trial lesson. Preferably a land away, so you then get 10 mins or so to chat to the instructor at the destination - see if you can get on with them, that is the biggest factor. Check they are not leaving imminently to join the airlines, in fact it is not a bad bet to find an instructor who has recently joined the school, as they should then be there for a couple of years and see you through to IMC if that is waht you want. Or find an old fashioned career instructor, rare these days, but not impossible.
School v Club? In general the school approach is more to teach courses, and should be effiecient at this, places like Cabair. Clubs may be more attractive for a more genial atmosphere, but can be a bit more lax on timing, availability etc. Both schools and clubs will arrange trips, it is in their interest to utilise the aircraft! I started at a club, then moved on to a school after 20 hours and found it better for me, though of course it is a personal choice. I had excellent instructors at both. It is never a bad thing to fly with other instructors - at about 12 hours I had a bad habit of over controlling the plane - a different instructor knocked this out of me very quickly and taught a valuable lesson!!!!
Choices of planes really come down to your budget - but four seaters can be more stable and forgiving than 2 seaters. The Piper PA38 (Tomahawk 2 seater) has a nickname Traumahawk, and the Cessna 150/152 2 seater is very tight on space if you are not skinny and/or short! Which leaves Cessna 172 and Piper PA28, both 4 seaters, good stable training platforms, strong and forgiving! The 172 is a high wing plane and slightly trickier to land, but downwards visibility is better than the PA28 which is low wing and so lands better with the aid of ground effect as you flare.
Also, if you learn on a 4 seater, you can take a friend (or preferably share the flying with another student at the same level as you) on club trips with instructor when learning of course!
I hope this helps, if you need any more info please ask or pm me